Francisella tularensis is the causative agent of the zoonotic disease tularemia. F. tularensis is a highly virulent intracellular pathogen that is easil transmitted to humans when aerosolized and thus has the potential for use as a bioterrorism agent. The molecular basis for the high infectivity and virulence of F. tularensis is not well understood. We have identified TolC as critical for the virulence of F. tularensis. TolC is an oute membrane channel protein involved in both multidrug efflux and the secretion of virulence factors by the type I protein secretion pathway. We have evidence that effector proteins secreted via TolC function to dampen host cell death responses during infection. This provides the bacteria extended time to replicate within the protected intracellular niche and prolongs survival within host tissues, thereby tipping the host-pathogen balance in favor of the pathogen. These host suppressive activities related to TolC function are likely key to the successful intracellular lifestyle and extreme virulence of F. tularensis. This proposal will generate critica new information about the mechanism of TolC in Francisella pathogenesis. The first aim of the proposal will define the contributions of TolC to the virulence of the human pathogenic F. tularensis Schu S4 strain. The second aim of the proposal will use complementary approaches to identify the virulence factors secreted via TolC. This proposal will not only lead to better knowledge of F. tularensis virulence mechanisms, but will also provide insights into mechanisms by which intracellular pathogens survive in the host and cause disease.